1. Field of the Invention
The present invention relates to a multi-carrier transmission system for conducting communications by dividing a transmission band into a plurality of sub-carriers and is one of several broadband wireless communications systems, and more specifically, relates to a transmitter, a receiver and a transmitting method for encoding in such a way that the peak power of a transmitting signal can be suppressed in a multi-carrier transmission system for conducting communications by a QPSK-modulation method for mapping each sub-carrier using a complex signal point expressed by two bits.
2. Description of the Related Art
In broadband wireless communications, frequency-selective fading due to multi-path degrading circuit quality is especially problematic. As a modulation method that has an excellent multi-path-proof fading characteristic, a multi-carrier transmission system shown in FIG. 21 is known. In this system, since a frequency-diversity effect can be obtained against frequency-selective fading by dividing a transmission band into a plurality of carriers (called “sub-carriers”) high-quality wireless transmission is possible. The orthogonal frequency-division multiplex (OFDM) technology shown in FIG. 22 is also one form of this system.
One of the problems of the multi-carrier technology is the increase of the peak power (or peak-to-average power ratio) of a transmitting signal. To compensate for the linearity of the system, a wide-range linear amplifier is needed. However, this amplifier is expensive and the power efficiency is low. If a cheap amplifier is used, non-linear distortion is caused by using a saturation region, and the characteristic degrades, which is a problem. For this reason, the technology cannot be put into practical use yet.
The solution of this problem is largely classified into two methods: (1) the restriction of input signals and (2) the restriction of output signals. The former method prevents a signal pattern in which peak power is increased by an encoding process, from occurring and causes no characteristic degradation. Furthermore, if those codes can extend the minimum distance, the receiving characteristic (bit error rate (BER)) can also be improved. The latter method, for example, by utilizing the fact that the occurrence probability of a signal pattern for generating a peak power is low, compulsorily cuts peak power using a specific threshold value when the peak power exceeds the threshold value, which corresponds to clipping and the like. This technology increases a side-lobe level due to non-linear distortion, that is, it causes inter-carrier interference. Thus, the latter method degrades the characteristic. Although there is a method for normalizing the entire envelope level of a signal into a threshold level, eventually the S/N is degraded. Therefore, the method also degrades the characteristic. To implement broadband and high-quality wireless transmission, the former method is recommended.
As a peak suppression code, a complementary code is well known, and the application of the code to a multi-carrier modulation system is being studied. This code is applicable to a multi-phase modulation (M-ary PSK (MPSK)). In the case of N sub-carriers, this code also provides an encoding rate of R=(log2N+1)/N, a minimum code distance of dmin(√(N/2)) d and a peak power amount of Pgain=2/N P(N). In this case, d and P(N) (=N2) represent a distance between signals and peak power in N sub-carriers, respectively. For example, in the case of four sub-carriers, R=¾, dmin=√2d and Pgain=1/2P(2), and in the case of eight sub-carriers, R=½, dmin=2d and Pgain=1/4P(4). Since the encoding rate decreases in proportion to the number of sub-carriers, the reduction of transmission efficiency cannot be avoided even if the improvement of error correction capability is taken into consideration. By replacing eight sub-carriers with two sets of four sub-carriers, an eight-sub-carrier system can be operated as two four-sub-carrier systems. However, even in this case, R≦¾ and the encoding rate cannot be further increased.
The encoding rate, minimum distance and peak power amount described above are disclosed, for example, in the following reference.
R. D. J. van Nee, “OFDM Codes for Peak-to-Average Power Reduction and Error Correction”, IEEE Globecom 96, London, p.740–744 (November 1996).
Since the encoding technology includes a non-linear operation, it is difficult to implement the technology by a logic circuit and an accordingly implementation method using a look-up-table is mainly adopted. Therefore, the encoding technology is not suited for high-speed signal processing, which is a problem.
An object of the present invention is to provide a transmitter, a receiver and a transmitting method for conducting high-performance wireless transmission by not using a signal point pattern which increases the peak power of a transmitting signal, as a signal point pattern which is the base of a multi-carrier transmitting signal and, for example, by performing communications encoding for suppressing the peak power to approximately 2 dB at a high encoding rate (for example, R=⅞) in view of the problem described above. Another object of the present invention is to realize a high speed by implementing the encoding by hardware.